This invention relates to an industrial-plant or commercial-building electrical power distribution circuit for connecting pulsed inductive loads to an AC source of electrical energy with a power factor correction capacitor or capacitors, and more particularly pertains to such a circuit having a harmonic filter for shunting non-linear-load generated harmonic currents away from the source of electrical energy without affecting the degree of power factor correction.
It is well known to connect, as illustrated in FIG. 1, a power factor correction capacitor 10 across a pair of power distribution lines 22 and 20 that supply electrical energy from a sinusoidal AC energy or power source to an inductive load. The inductive load may be a single load or as shown here a composite load made up of the loads 24, 26 and 28 such as motors, fluorescent lighting with inductive ballasts, and solenoids. The voltage supplied to an industrial plant from a utility company AC power source is usually reduced by a step-down transformer 16 having output terminals 12 and 14 to which the plant power distribution lines or conductors 20 and 22 are connected.
The power factor, PF, of a distribution circuit as viewed at terminals 12 and 14 (and as essentially seen by the power company) is ##EQU1## which is equal in turn to the cosine of the angle by which the line current (e.g. at terminal 16) lags the line voltage across terminals 12 and 14. KW (kilowatts) stands for the real power consumed by the resistive portion of the load whereas KVA (kilovolt-amps) is the product of the in-phase current and AC line voltage supplied by the transformer 16 to distribution lines 22 and 20. The transformer 16 is conservatively rated in KVAs to deliver a somewhat greater power than the actual plant load. As the capacitance of a power factor capacitor increases the more the leading current flows tending to increase the power factor. At a power factor of exactly 1.00 no leading or lagging current is supplied by the AC source.
Power factor correction capacitors are sometimes rated for given line voltage in KVAR's, i.e. the product of the rated line voltage at terminals 12 and 14, and the leading reactive current that flows through the capacitor 10. Thus the KVAR rating of a power factor correction capacitor is ##EQU2## where f is the frequency of the source voltage and C is the capacitance of the power factor capacitor.
As used herein, the name plate KVAR rating of a capacitor, as determined by the formula given above wherein f is the name plate frequency of the AC energy source, C is the name plate capacitance, and KV is the name plate service voltage at which the capacitor is intended to operate.
Electrical power engineers often speak of a power factor correction capacitor as a "reactive leading current generator" or a "KVAR generator". It follows from the above that when the service voltage rating and the KVAR rating of a power factor correction capacitor are known, then so is the maximum safe current (I.sub.m) rating of the capacitor known: ##EQU3##
It is often preferable to connect a separate power factor correction capacitor (not shown) across the distribution line physically adjacent to each inductive load thus correcting the power factor right at each load. To obtain the same power factor as seen at the terminals of the AC energy source, all the individual-load power factor correction capacitors taken together will provide the same total leading KVAR's as the single power factor correction capacitor 10 in FIG. 1.
In recent years, the increasing use of semiconductor circuit industrial motor controllers presents to the power line a nonlinear impedance. Such nonlinear loads trigger and sustain large harmonic currents in the power distribution system with a power factor correction capacitor at one or more odd multiples of the AC power line frequency. This is true because the power factor correction capacitor and inductance of the AC source, including the step-down transformer, are connected mutually in parallel, tend to be broadly resonant, and exhibit a high impedance, at a frequency in the range of from the 5th to the 13th harmonic of the AC line frequency.
For example in a three phase distribution system a "six-pulse" motor controller generates a large 5th harmonic current and a not quite so large a 7th harmonic current. The more refined "twelve-pulse" motor controller or AC to DC converter load produces a strong 11th harmonic current and not quite so strong a 13th harmonic current. Such harmonic currents raise the total current in the distribution conductors, causing IR drops, distorting the line voltage, blowing fuses and/or throwing circuit breakers, and causing deleterious heating of the plant step-down transformer. Among further disadvantageous consequences are costly plant down time unless heavier distribution conductors, and transformers of higher KVA rating are provided. More leading current yet will of course reduce the power factor again to less than unity.
The conventional solution to this problem has become the substitution for the power factor correction capacitor 10 of FIG. 1 by a combination of a power factor correction capacitor in series with an inductor whereby this series branch circuit serves both to shunt the dominant harmonic current away from the AC energy source and at the lower frequency, namely that of the AC power source, to correct the power factor of the distribution system. However it has not proved practical to make the resonant frequency of that series tuned circuit exactly the frequency of the dominant harmonic because the capacitor must carry all of the power factor correction current at the AC source frequency and all of the higher-frequency harmonic current generated by the non-linear load.
In this power distribution circuit shown in FIG. 2, all of the elements are the same as those in the power distribution circuit of FIG. 1 except that the power factor capacitor 10 of FIG. 1 is replaced by a power factor correction capacitor 30 connected in series with an inductor 32. The values of inductance and capacitance of the series circuit capacitor 30 and inductor 32 are resonant at near the 5th harmonic frequency of the AC power source, namely at 300 HZ when the AC line frequency is 60 Hz. The impedance of this series LC circuit is low at the frequency near its resonant frequency and is strongly leading reactive, i.e. capacitive, at the frequency of the AC power source. The impedance of this series circuit is relatively high at the lower frequency of the AC energy source and the capacitor 30 generates the KVARS needed to raise the power factor toward unity while at the dominant harmonic of the AC source frequency this series LC circuit acts to shunt essentially all of the harmonic current generated by non-linearities in the load 24, 26, and/or 28 away from the AC power source.
When the capacitor 30 and the inductor 32 are made resonant exactly at the dominant harmonic, the total current through capacitor 30 and inductor 32 is at a maximum and the KVAR rating, especially the current rating, of each must be many times greater than that of the capacitor 10 in FIG. 1 which they replaced.
It follows that the power factor correction capacitor 30 must be significantly larger and more expensive than capacitor 10 in FIG. 1 especially to carry both the harmonic current in addition to the leading current at the frequency of the AC power source. Capacitor 30 must be larger still because at the frequency of the AC power source the series inductance (inductor 32) produces some lagging current itself, and because the inductor 32 causes the capacitor voltage to be much greater than the line voltage. In fact capacitor 30 in the exactly series harmonic tuned LC circuit must be many times more expensive than the power factor correction capacitor 10 that it replaced.
Power factor correction capacitors for use in the circuits of FIGS. 1 and 2 have been manufactured and their performance to cost ratio continuously improved over a period of more than fifty years and are today a commodity readily available to plant engineers from many manufacturers with the most commonly used plant voltage ratings, e.g. 240, 480, and 600 volts, and with a wide range of available KVAR ratings. These capacitors are required to meet the current ANSI Standard No. 18 and the IEEE Standard for Shunt Power Capacitors and typically consist of many rolled capacitor sections connected in parallel, each section having metallized plastic film electrodes and a plastic dielectric sheet. The parallel connected rolled sections are surrounded by a casing and impregnated by a dielectric liquid that enhances capacitance and raises the safe operating voltage. But these commercial capacitors are all limited in current to about 100% of that which corresponds to its name plate KVAR rating (at AC source frequency). Capacitors of a different construction are manufactured for use as SCR commutating capacitors that provide a much higher current carrying capacity. These high rated current capacitors employ metal foil electrodes and have a stable capacitance in service. Such a capacitor could have been used in the series circuit of FIG. 2 permitting tuning to exactly the dominant harmonic, but only at a very much greater expense, e.g. $100,000 rather than $25,000 for a capacitor rated at 800 KVAR at 60 HZ.
It is therefore conventional practice to employ the less costly metallized film capacitor with a more modest current carrying capacity, and to detune the series LC circuit of FIG. 2 to a frequency lower than that of the harmonic current to be shunted away from the power source. Another reason for detuning is that the lower cost standard power-factor-correction capacitor described above has a self healing feature whereby when voltage breakdown occurs at a weak point in the plastic dielectric, the metallized film in an area surrounding the fault is vaporized thus clearing the fault; but each such breakdown and healing point causes a gradual reduction in time in the total capacity and thus a detuning in the filter of FIG. 2.
An example of what is done in practice when the fifth harmonic is to be shunted by the series LC circuit of FIG. 2, is to make the resonant frequency of the LC series circuit 4.7 times, instead of exactly 5 times, the frequency of the AC energy source, substantially reducing the harmonic current through the capacitor to the available capacitor rating but at the same time substantially reducing the efficacy of the harmonic shunt.
It is therefore an object of the present invention to provide a lower cost electrical power distribution circuit, with a power factor correction capacitor and means for harmonic current suppression that overcomes the above-noted shortcomings of the prior art.
It is further an object of this invention to provide such an electrical power distribution circuit having greater harmonic current filtering efficiency.